North Korean leader Kim Jong-un says his scientists have successfully miniaturised thermo-nuclear warheads to place on a ballistic missile and create a "true" deterrent, state media reports.

Key points:

North Korea claims nuclear breakthrough

Kim Jong-un says warheads are "thermo-nuclear"

Doubts over North's true nuclear abilities

While Pyongyang has talked of success in mastering miniaturisation before, this is the first time Mr Kim has so explicitly claimed a breakthrough that experts see as a game-changing step for the North's nuclear capabilities.

"The nuclear warheads have been standardised to be fit for ballistic missiles by miniaturising them," Mr Kim noted during a meeting with nuclear technicians, the North's official KCNA news agency said.

Mr Kim echoed the threat, warning that North Korea would "never hesitate to make a pre-emptive nuclear strike" in the event of any effort by the US and its allies to undermine its national sovereignty as a nuclear state.

Experts divided on North Korea's capabilities

Experts have been divided on just how far North Korea may have got in its ability to miniaturise nuclear warheads, although several top US and South Korean military officials have briefed on its likely success.

While North Korea is known to have a small stockpile of nuclear weapons, its ability to deliver them accurately to a chosen target has been in doubt.

Mr Kim's confirmation still leaves a question mark over the North's ballistic missile capabilities, with many experts believing it is years from developing a working inter-continental ballistic missile that could strike the continental United States.

It is also unclear whether any miniaturised device the North has designed would be robust enough to survive the shock, vibration and temperature change associated with ballistic flight.

North Korea's claim to have successfully tested an H-bomb in January was greeted with scepticism at the time as the estimated yield was seen as far too low for a full-fledged thermo-nuclear device.

However, numerous weapons experts have suggested it may have been a "boosted" fission device, which makes more efficient use of nuclear material and can be made smaller without sacrificing yield.